For 100 years, the turbid waters off the coast of Long Island had kept secret the fate of the USS San Diego. Its wreckage slept amid schools of sea bass and scurrying lobsters, 100 feet deep off the coast of Fire Island.

Tuesday, Navy researchers and oceanographers from the University of Delaware announced that the 500-foot armored cruiser was done in by a naval mine, dispensed by a German U-boat, U-156, that had lurked 8 miles off the coast.

The announcement, made at the American Geophysical Union’s fall convention in Washington, D.C., was based on years of research that culminated with four underwater visits to the shipwreck since 2017. Crew from the U.S. Coast Guard’s Fire Island station helped commemorate the 100th anniversary of the shipwreck in July, placing a wreath there.

The July 19, 1918, sinking — which was the only loss of a U.S. Navy ship in World War I — was complete just 28 minutes after the exploding mine tore into a coal-filled storage hold, sending the cruiser to the bottom southeast of the Long Island shore.

“The sinking happened in minutes, despite the precautions and the fact that underwater explosive attacks were rare at the time,” said Ken Nahshon, an engineer with the Maryland-based Naval Surface Warfare Center Carderock Division.

The sinking came as the San Diego was bound from Portsmouth, New Hampshire, to New York to join a convoy delivering troops and war material to France. The United States had declared war on Germany the previous year, on April 6.

Nahshon said the ship’s captain had taken several precautions because of the presence of German submarines in the area. He had stationed 17 seamen to watch for the telltale bubble trails left by torpedoes. He took a zig-zag course to avoid being targeted. And he ordered the ship’s water-tight doors sealed to limit the inrush of water should the ship fall victim to an attack.

But a mysterious explosion rocked the ship's port side in broad daylight, piercing its side below the 6-inch-thick steel plating meant to protect the ship from artillery fire. The inrushing sea caused the boat to list, flooding its gun decks. Hundreds of tons of sea water were able to reach deep into the ship’s interior via chutes used to feed coal to the boiler room, causing the ship to heel further over. Its fate was sealed.

With the ship’s radio out, sailors were able to raise an alarm only after rowing a lifeboat to the Long Island hamlet Point O’Woods.

That drew a flotilla of merchant craft speeding to the rescue — despite fears that the submarine might still be lurking — and they began plucking sailors from the waters before the U.S. Navy arrived. Of the ship’s 1,100-member crew, only six perished.

A description written just days after the disaster by Capt. Harley Hannibal Christy, the San Diego’s commander, said the seas had been calm and the visibility good when the fatal blow occurred just after 11 a.m.

“The explosion felt like a dull heavy thud; it lifted the stern slightly and shook the ship moderately fore and aft ….” he wrote in a July 20, 1918 report catalogued at the National Archive. “During this time, the behavior of the ship did not convince me she was in much danger of sinking.”

But the 2,900 tons of coal that were to have fueled its trans-Atlantic passage — much of it loaded on decks above the waterline, made the San Diego top-heavy, and it soon capsized. Three coastal steamers — the Malden, Bussum and the F. P. Jones — helped rescue seamen. News reports later said nearly 400 clamored aboard the Malden — many who had clung to makeshift rafts or had been kept afloat by life vests. More than 700 found refuge aboard the Bussum, an oil tanker bound from Baltimore to Boston.

“They came to our rescue without hesitation and picked up the survivors although they ran serious risk of being torpedoed in case a submarine had been in the vicinity,” Christy wrote.

Fear quickly spread speculation across Long Island that the ship might have fallen victim to a German spy who might have buried a bomb in the coal loaded aboard.

Decades passed with no clear answer as to what had sent the 10-year-old warship to the bottom. With 3,000 Navy shipwrecks and more than 14,000 military planes then under the Navy’s purview, the San Diego did not draw much official attention by Navy researchers.

The boat was mostly left to the amusement of recreational divers drawn by its historical significance, and sportsmen who fished for sea bass and other species near the mussel-encrusted hulk.

That began to change about 20 years ago, as this year’s 100th anniversary of the end of WWI began to draw near.

Researchers under the direction of the Naval History and Heritage Command sought to determine whether the San Diego’s sinking was by sabotage, accident, enemy torpedo or enemy mine.

Using underwater robotic tools and remote instruments, researchers created a three-dimensional sonar model of the wreck, which came to rest upside down in waters within sight of the 168-foot-high Fire Island lighthouse.

They concluded that the craft had been hit with a single explosion that overwhelmed the ship’s ability to limit the inrushing waters.

“The captain did everything right,” said marine archaeologist Alexis Catsambis, one of the researchers.

“We believe U-156 sank San Diego,” Catsambis said. “And we believe it used a mine to do so.”

One researcher, a University of Delaware marine geologist whose grandfather was preparing to sail to Europe aboard a similar convoy when World War I ended, said he felt a reverence while working at the wreck site.

“I could see the shoreline of Long Island in the distance, and it was palpable to realize the war was not just ‘Over There’ as people would sing -- the war was here too,” ArthurTrembanis said.

“The site is haunting and eerie, like the poppy fields of Flanders,” he said. “But on the seabed, it’s not poppies, but tentacled anemones and teeming fish that are transforming it into a living scene.”